CAMPBELL AND REECECHAPTER 3

Water & Life

Hydrogen Bonding

2 characteristics contribute to H-bonds

1. polar covalent bond between H & O

2. molecule is bent (V-shaped)

Water in Liquid State

H-bonds ~ 1/20th as strong as covalent bond

bonds constantly being formed/broken (each lasts a few trillionth of a sec.)

@ any given moment a substantial % of all water molecules are part of a H-bond

H-bonds in Water

4 Emergent Properties of Water

all 4 contribute to suitability for life on Earth

1. Cohesion2. Moderation of Temperature by

Water3. Ice Floats4. Water is Universal Solvent

Cohesion of Water Molecules

H-bonds hold water molecules together = cohesion (like molecules sticking together)

contributes (with adhesion) to the transport of water & nutrients against gravity in plants

Water & Nutrients Move up the Xylem

Surface Tension

a measure of how difficult it is to stretch or break the surface of a liquid

Water has greater surface tension than most other liquids due to H-bonding

Moderation of Temperature by Water

Water moderates temperature by absorbing heat from air that is warmer and releasing stored heat energy to air that is cooler.

Water can absorb or release a relatively large amount of heat with only a slight change in its own temperature.

Citrus Trees Sprayed with Water to Prevent Frost Damage

Heat & Temperature

heat: a measure of matter’s total KE due to motion of its molecules (so heat depends on volume of matter)

temperature: a measure of heat intensity; represents the average KE of its molecules regardless of volume

Calorie

the amt of heat it takes to raise the temperature of 1 g water 1 ºC

orthe amt of heat that 1 g water releases when it cools by 1ºC

Kilocalories

a kilocalorie (kcal): quantity of heat required to raise temperature of 1,000 g of water by 1ºC

Food packaging calls a kcal “calorie”

1 joule (J) = 0.239 cal1 cal = 4.184 J

Specific Heat

is the amt of heat that must be absorbed or lost for 1 g of the substance to change its temperature by 1ºC

specific heat of water is high which explains its ability to stabilize temperature

Specific Heat of Water

1 cal/g x ºC specific heat of water high compared to other materials, water changes its temperature less when it absorbs or loses a given amt of heat

Specific Heat of Water

due to H-bondingabsorbed heat must 1st break all the

H-bonds b/4 it increases the average KE of its molecules

when temperature drops slightly, many additional H-bonds form releases additional energy in form of heat

Water’s High Specific Heat

allows water to absorb and store a large amt of heat from Sun & warm up only a few degrees

@ night & during winter the gradually cooling water warms the air

Water’s High Specific Heat

the water that covers most of Earth keeps temperature fluctuations on land & water w/in limits to permit life on Earth

Moderating Effects of Large Bodies of Water on Temperatures

Evaporative Cooling

temperature is average KE of molecules

some evaporization of water occurs @ any temp because some individual molecules of water my gain enough KE to gas phase

Heat of Vaporization

= quantity of heat a liquid must absorb for 1 g liquid phase gas phase

water has high heat of vaporization relative to most other liquids because of H-bonding: absorbed energy must 1st break H-bonds, then increase KE of molecules

High Heat of Vaporization of Water

Global scale: helps moderate Earth’s climate

Evaporative Cooling

= as liquid evaporates the surface of the liquid that remains cools down

contributes to stability of temperature in lakes & ponds

provides mechanism to keep terrestrial organisms from overheating

Evaporative Cooling

Ice Floats

water one of few substances that is less dense in its solid phase than in its liquid phase

water expands when goes from liquid solid

> 4ºC water acts like other liquids: expands as it warms up, contracting as it cools

Ice Floats

important factor in suitability of Earth’s environment to life

if ice sank: lakes & ponds (even oceans) could

freeze solid (from bottom to top)with summer temps only top upper

meter or so would melt

Ice Floats

so ice insulates water below it, preventing it from freezing & allowing life to exist under frozen surface

Global Warming

average air temperature in Arctic has increased by 1.4ºC in the 50 years from 1961 -2011

ice forms later in fall and melts earlier to cover smaller area

Water: Solvent of Life

aqueous solution: one in which water is the solvent

water is best solvent on Earth because …

water is

polar

Water as Solvent

salt in water dissociates and each cation and anion is surrounded by water molecules called a hydration shell

Water as Solvent

sea water & cytosol in cells contain variety of ions all in hydration shells

polar molecules dissolve in water sugarsproteins with ionic or polar regions on

surface

Hydrophilic Substances

any substance that has an affinity for water

substances too large to dissolve but still hydrophilic remain suspended in aqueous liquid (a colloid)

Hydrophobic Substances

nonionic, nonpolar substances repel water

Molecular Mass/Moles

sum of masses of all the atoms in a molecule

not practical to use #s of molecules so use moles (mol)

1 mole = 6.02 x 10²³

Molar Mass

molecular mass of glucose is 144 daltons

6.02 x 10²³ daltons in 1 gso molecular mass of glucose can be written as 144g in 1 mole of glucose

measuring in moles allows you to combine substances in fixed ratios of molecules

Molarity

1 mole of glucose in 1 liter water = 1-molar or 1M solution of glucose

Molarity: the # of moles of solute per liter of water (for aqueous solutions)

Water on Mars

Astrobiologists have looked for life elsewhere based on: Is water present?

Mars has polar ice caps, ice under surface and enough water vapor in atmosphere to form frost still looking for life….

Dissociation of Water

Dissociation of Water Molecules

only 1/554 million molecules of water are dissociated (in pure water)

@ 25ºC the concentration of H+ or OH-

is 10-¹ Mconcentration of H+ or OH- is very low but they are each very reactive

pH Scale

Acid a substance that increases the [H+]overall has more H+ than OH-

Base a substance that reduces the [H+]overall has more OH- than H+

pH Scale

bases:1. reduce [H+] by accepting H+

or2. reduce [H+] indirectly by

contributing OH-

Weak Acids Form Reversible Reactions

pH Scale

In any aqueous solution @ 25ºC, the product of [H+] x [OH-] = 10ˉ¹

the pH scale compresses the range of H+ and OHˉ concentrations by using logarithms

pH = the negative logarithm of [H+] or pH = -log [H+]

pH Scale

pH < 7.0 = acidpH > 7.0 = basemost biological solutions are pH 6-8exceptions: stomach acid is pH

Each pH unit = 10x differences in [H+] or [OHˉ] (pH of 4 has 100x more H+ than pH of 6)

Buffers

molecules that are able to resist changes in pH

consists of an acid-base pair that combines reversibly with H+ when H+ in excess & donate H+ when they have been depleted

Buffers

Buffers

Acidification

burning fossil fuels

increases CO2 in atmosphere

CO2 dissolves in water

fresh water & sea water pH decreases

Acidification of Oceans

Acid Precipitation

rain, snow, sleet, fog with a pH < 5.2normal rain has pH 5.2adversely affects: 1. life in ponds & streams2. land plants3. soil chemistry

Acid Precipitation